Piezoelectric vibrators are used as a device to implement a timer function in electronic apparatuses. As electronic apparatuses become smaller in size, piezoelectric vibrators are required to be smaller in size as well. As such, a piezoelectric vibrator manufactured by using the MEMS (Micro Electro Mechanical Systems) technique (hereinafter, referred to as “MEMS vibrator”) is drawing attention today.
As a MEMS vibrator, there exists a configuration in which one vibrator and another vibrator are made to vibrate with mutually reverse phases. For example, Patent Document 1 discloses a MEMS vibrator having three vibration arms. In the stated MEMS vibrator, a vibration arm in the center and two vibration arms in the outer side portions perform bending vibrations with mutually reverse phases by causing a direction of an electric field applied to the vibration arm in the center and a direction of an electric field applied to the two vibration arms in the outer side portions to be opposite to each other.
Patent Document 1: International Publication No. WO 2008/043727.
For typical configurations in which directions of electric fields applied to vibration portions are opposite to each other, there exists a parallel connection configuration and a series connection configuration. FIG. 18 is a diagram illustrating an example of electric connection in a parallel connection configuration. Meanwhile, FIG. 19 is a diagram illustrating an example of electric connection in a series connection. For a typical configuration of a MEMS vibrator, three or more vibration arms are included in the configuration, as disclosed in Patent Document 1. However, in FIGS. 18 and 19, in order to simplify the explanation, only two vibration arms are illustrated. It is noted that FIGS. 18 and 19 are diagrams each illustrating an electrical connection relationship among a silicon layer, an upper electrode, a lower electrode, an insulation layer, and a piezoelectric layer in the two vibration arms, and that they are not actual cross-sectional views.
First, the parallel connection configuration will be described. As shown in FIG. 18, a MEMS vibrator 1800 includes a silicon layer 1810, an insulation layer 1811, and vibration arms 1812 and 1813. The vibration arm 1812 includes an upper electrode 1820, a lower electrode 1821, and a piezoelectric layer 1822. Likewise, the vibration arm 1813 includes an upper electrode 1830, a lower electrode 1831, and a piezoelectric layer 1832. A potential of each electrode is controlled so that the direction of an electric field applied to the vibration arm 1812 and the direction of an electric field applied to the vibration arm 1813 are opposite to each other. In this parallel connection configuration, combined capacitance Cf of the MEMS vibrator 1800 equals the sum of electrostatic capacity Ca1 of the vibration arm 1812 and electrostatic capacity Ca2 of the vibration arm 1813 (Cf=Ca1+Ca2). As such, this configuration has an advantage that the combined capacitance Cf can be made large.
It is noted that in the parallel connection configuration, a potential of the lower electrode 1821 of the vibration arm 1812 and a potential of the lower electrode 1831 of the vibration arm 1813 are different. Because resistivity of the silicon layer 1810 is low, the lower electrodes 1821 and 1831 are short-circuited if they are directly disposed on the silicon layer 1810. As such, the insulation layer 1811 is provided between the silicon layer 1810 and the lower electrodes 1821, 1831. This causes degradation in characteristics in some case because of generation of stray capacitance Cs formed with stray capacitance Cb1 between the lower electrode 1821 and the silicon layer 1810 and stray capacitance Cb2 between the lower electrode 1831 and the silicon layer 1810 (Cs=1/(1/Cb1+1/Cb2)).
Next, the series connection configuration will be described. As shown in FIG. 19, a MEMS vibrator 1900 includes the silicon layer 1810 and the vibration arms 1812 and 1813. A potential of each electrode is controlled so that the direction of an electric field applied to the vibration arm 1812 and the direction of an electric field applied to the vibration arm 1813 are opposite to each other. In the configuration shown in FIG. 19, the lower electrode 1821 of the vibration arm 1812 and the lower electrode 1831 of the vibration arm 1813 are floating electrodes and have the same potential. Accordingly, unlike the MEMS vibrator 1800 shown in FIG. 18, it is unnecessary to provide the insulation layer 1811, thereby receiving no influence of the stray capacitance Cs. Note that, however, combined capacitance Cf (=1/(1/Ca1+1/Ca2)) of the MEMS vibrator 1900 is smaller than that of the parallel connection configuration. This increases resonant impedance and causes degradation in characteristics in some case.